diff --git a/lib/mongo_ecto/conversions.ex b/lib/mongo_ecto/conversions.ex
index c32a385..4aaeb25 100644
--- a/lib/mongo_ecto/conversions.ex
+++ b/lib/mongo_ecto/conversions.ex
@@ -74,13 +74,25 @@ defmodule Mongo.Ecto.Conversions do
 
   defp document(doc, pk) do
     map(doc, fn {key, value} ->
-      pair(key, value, pk, &from_ecto_pk(&1, pk))
+      # Recurse with `nil` so the parent schema's primary key isn't
+      # accidentally rewritten when it appears inside a nested document
+      # (e.g. an `embeds_one` whose embedded schema has its own `:id`
+      # field). Without this guard, an embed like
+      #
+      #   %{jurisdiction: %{id: "MD", title: "Maryland"}}
+      #
+      # gets serialized as `jurisdiction._id`, which breaks queries that
+      # filter on `jurisdiction.id`. The pk → _id remap is only meaningful
+      # at the top of the document.
+      pair(key, value, pk, &from_ecto_pk(&1, nil))
     end)
   end
 
   defp document(doc, params, pk) do
     map(doc, fn {key, value} ->
-      pair(key, value, pk, &inject_params(&1, params, pk))
+      # Same rationale as `document/2`: scope the pk → _id rename to the
+      # outer document. See the comment in `document/2`.
+      pair(key, value, pk, &inject_params(&1, params, nil))
     end)
   end
 
diff --git a/test/mongo_ecto/conversions_test.exs b/test/mongo_ecto/conversions_test.exs
new file mode 100644
index 0000000..c933ee8
--- /dev/null
+++ b/test/mongo_ecto/conversions_test.exs
@@ -0,0 +1,72 @@
+defmodule Mongo.Ecto.ConversionsTest do
+  use ExUnit.Case, async: true
+
+  alias Mongo.Ecto.Conversions
+
+  describe "from_ecto_pk/2" do
+    test "renames the primary key to :_id at the top level" do
+      assert {:ok, encoded} = Conversions.from_ecto_pk(%{id: "abc", title: "t"}, :id)
+
+      assert Map.new(encoded) == %{_id: "abc", title: "t"}
+    end
+
+    test "does not rename :id inside a nested map when the parent pk is :id" do
+      input = %{
+        id: "abc",
+        title: "outer",
+        jurisdiction: %{id: "MD", title: "Maryland"}
+      }
+
+      assert {:ok, encoded} = Conversions.from_ecto_pk(input, :id)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:_id] == "abc"
+      refute Map.has_key?(encoded_map, :id)
+
+      # The nested :id field must be passed through unchanged — it belongs to
+      # the embedded sub-document, not the parent schema.
+      assert Map.new(encoded_map[:jurisdiction]) == %{id: "MD", title: "Maryland"}
+    end
+
+    test "does not rename :id inside a list of nested maps" do
+      input = %{id: "abc", items: [%{id: "one"}, %{id: "two"}]}
+
+      assert {:ok, encoded} = Conversions.from_ecto_pk(input, :id)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:_id] == "abc"
+
+      assert Enum.map(encoded_map[:items], &Map.new/1) == [
+               %{id: "one"},
+               %{id: "two"}
+             ]
+    end
+
+    test "leaves nested maps alone when the pk is nil" do
+      input = %{id: "abc", jurisdiction: %{id: "MD"}}
+
+      assert {:ok, encoded} = Conversions.from_ecto_pk(input, nil)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:id] == "abc"
+      assert Map.new(encoded_map[:jurisdiction]) == %{id: "MD"}
+    end
+  end
+
+  describe "inject_params/3" do
+    test "renames the primary key to :_id only at the top level" do
+      input = %{
+        id: "abc",
+        jurisdiction: %{id: "MD", title: "Maryland"}
+      }
+
+      assert {:ok, encoded} = Conversions.inject_params(input, {}, :id)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:_id] == "abc"
+      refute Map.has_key?(encoded_map, :id)
+
+      assert Map.new(encoded_map[:jurisdiction]) == %{id: "MD", title: "Maryland"}
+    end
+  end
+end